“Nanowebs” are nonwoven webs comprising primarily, or even exclusively, fibers that have a number average diameter of less than one micrometer. Due to their extremely small pore dimensions and high surface area to volume ratio, nanowebs have been expected to be utilized as substrates for many applications such as, for example, high performance air filtration, waste water filtration, filtration membranes for biological contaminants, separators for batteries and other energy storage devices. However, one disadvantage of nanowebs for these applications is their poor mechanical integrity.
The number average diameter of nanofibers are less than 1000 nm and sometimes as small as 20 nm. In this dimension, even if they are layered and formed as thick membranes, the mechanical strength of the resulting structures is not sufficient to withstand macroscopic impacts for filtration applications such as normal liquid or air flows passing through them or higher strength required for winding and handling during end use manufacturing steps. Nanowebs made for example by electrospinning or electroblowing also tend to have low solids volume content (solidity), typically less than about 20%.
Unsupported nanowebs also exhibit an excessive reduction in width (“necking”) when tension is applied in the machine direction (MD), such as when winding or post processing, for example, when applying surface treatments and laminating for some product applications. Where the material is unwound and wound again, varying tensions can result in different widths and potentially create variations in sheet properties. A material is desired which is more robust with regard to applied tension.
The low surface stability of electrospun and electroblown nanowebs also creates problems when handling the sheet or when the sheet goes over rolls or other surfaces. Fibers are removed from the sheet and collect on various contact surfaces, such as hands, rolls, guides, etc., and sheet properties can be potentially changed and/or process equipment contaminated with fibers. A material with a more stable surface is desired.
The open structure of “as-spun” nanowebs typically yields structures with a solidity in the range of 20% to 10% or even lower. This open structure provides low resistance to fluid flow and/or ion flow due to the low solidity, which conversely can be reported as large total pore volume percent or “porosity”. Typically, as-spun nanowebs have maximum pore sizes between fibers of in the range of about 0.5 to 10 micrometers, even as high as 20 micrometers, and mean flow pore size ranges between about 0.05 to 10 micrometers.
Some fabric applications require smaller pore size and hence higher web solidity, approaching or even within the 40% to 90% range. These fabrics exhibit higher filtration efficiency and in general better overall barrier properties for fluids, and resistance to “short-circuit” in battery separator and other energy storage applications. Other applications require low air flow and low liquid flow while yielding a low resistance to moisture vapor transmission and require the higher % solidity materials or small pores. Conventional nanowebs are currently excluded from these applications that require higher solidity, since there is typically not enough nanoweb material to modify to smaller pore size and higher solidity. Instead, nonwoven webs which can be produced in commercially acceptable sizes and basis weights, such as meltblown webs, are often used in such applications. However, meltblown webs consist of much larger fibers, typically between about 2 to 10 micrometers in diameter, and modification of as-spun meltblown webs to meet the small pore size limitations necessary for high filtration efficiency requires high solidifies, even as high as about 80%, and results in dramatic decreases in fluid flow rates through such modified meltblown webs.
“As-spun” nanowebs also exhibit relatively high surface drag or friction, a surface coefficient of friction as high as about 2.5. Some material applications require smoother or softer, low friction hand. Other applications require a smooth outer surface for filter cake release, or low liquid flow resistance. For a material to be used in these applications, it requires a “smooth” surface which promotes low friction and high wear resistance.
It is known that physical properties of a web can be improved by calendering, which is the process of passing a sheet material through a nip between rolls or plates to impart a smooth, glossy appearance to the sheet material or otherwise improving selected physical properties.
Through the calendering of paper or other fibrous materials, an effort is made to further improve the quality of paper formed or, in providing a standard level of quality, to achieve a higher running speed or increased bulk of the paper being produced. It is well known that the plasticity or molding tendency of paper or fiber may be increased by raising the temperature and/or the plasticizer content of the paper or fiber. A considerable change in mechanical properties, including plasticity, occurs when the temperature of the polymers contained in the paper rises to or beyond the so-called glass transition temperature (Tg), at which point the material may then be more readily molded or formed or finished than it can below that temperature.
The prior art discloses various methods and apparatus for confining the deformation of web fibers to only the surface portions of the web. U.S. Pat. No. 4,606,264 provides a method and apparatus for temperature gradient calendering, wherein paper or like material is passed into at least one nip formed by an iron roll and a soft roll. The iron roll is heated to at least the temperature at which the fibers in the web begin to deform; for paper, that temperature is approximately 350° F. As therein disclosed, it is preferred that the web is passed through two successive nips, one for glazing one face of the web and the other for glazing the opposite face.
Many nonwoven fabrics are interfilamentarily bonded to impart integrity to the fabric. While there are several bonding techniques available, thermal bonding processes prevail in the nonwovens industry both in volume and time devoted to the research and development of new products. These processes have gained wide acceptance due to simplicity and many advantages over traditional chemical bonding methods. Attractive features include low energy and raw material costs, increased production rates, and product versatility. Chemical simplification, since adhesive binders are not used, reduces concerns over the environment. U.S. Pat. No. 4,035,219 and U.S. Pat. No. 5,424,115 provide examples of point bonding of nonwoven webs to enhance physical properties.
U.S. Pat. No. 2,277,049 to Reed introduced the idea of using fusible fibers to make nonwoven fabrics by blending fusible and nonfusible fibers of similar denier and cut length and treating the web with either solvent or heat. The fusible fibers become tacky and act as a binder. A nonwoven fabric results after pressing and cooling the tacky web.
However, the use of temperatures near the melting point (™) of the fiber in a nanoweb is detrimental to the quality of the web. The small size of the fibers combined with the uneven heating inherent in calendering machinery tend to produce uneven melting and bonding and render the web less effective for filtration and battery separator and other energy storage applications. The deficiency in the prior art in the area of strengthening of webs of low basis weight and comprising fine denier fiber is exemplified in EP 1 042 549, in which thermal bonding in a pattern is used to produce a less deformable web. The factor Poisson's Ratio times basis weight (in ounces per square yard) is disclosed to be limited to less than 1.2, but Poisson's Ratios of the order of 2.5 to 4 are exemplified. Similarly in U.S. Pat. No. 5,858,515, a bonding pattern is described that strengthens a web but at the cost of a considerable reduction in open area or porosity.
However, these prior art methods are neither concerned with nor directed to the stabilization of nanoweb structures, and the delicate nature of nanowebs has heretofore prevented application of these techniques to their stabilization. Application of temperatures that melt or deform such fine webs results essentially in the destruction of the web fiber network.
There is therefore a need for a process to strengthen nanowebs while retaining their porosity and uniformity.
The present invention is directed to increasing the utility of nanowebs by improving their physical properties while maintaining a high open area and hence porosity, and to the webs produced thereby.